Auth Documentation: update with walkthroughs for setting up identity with cloud providers

### Issue type

**I'm submitting a ...** (check one with "x")

* [ ] bug report
* [x] feature request

### Issue description

Attempted to use OAuth Google example for setting up authentication with Azure AD B2C (also based on OAuth), but involved a lot of extra configuration and overwriting of auth code. As Nebular develops would be great to add more examples for more providers, like Facebook, Azure AD B2C and other cloud providers AUthentication services as a quick way for people to get a dashboard up and running.

Further clarity of how to change the dummyprovider over to the other auth methods in the ngx-admin template would be a good addition, so that people starting with that as a code base can easily understand how to modify the authentication in a walkthrough style.